Such a hydraulic antivibration support is disclosed, for example, in document EP-A-0 547 287.
The antivibration support disclosed in that document nevertheless suffers from the drawback that the flap is always free to vibrate between its two abutments, even when the central nozzle is open.
Unfortunately, experience shows that that phenomenon spoils the operating efficiency of the central nozzle: in other words, when the central nozzle is opened essentially for the purpose of reducing the elastic stiffness of the antivibration support, in particular when the engine is idling, the fact that the flap remains free to vibrate in this situation prevents the elastic stiffness of the support from decreasing as much as it could if the flap were prevented from moving.
Also, document EP-A-0 297 974 describes a hydraulic antivibration support in which provision is made to lock the decoupling flap at the same time as a nozzle is opened between the working chamber and the compensation chamber.
That hydraulic antivibration support operates in entirely satisfactory manner, but suffers from the drawback of using relatively complex control means.
Also, the control means in question are mounted on the side of the hydraulic antivibration support, thereby increasing the size of the support and making it considerably more complicated to assembly and to fill.